Cars already have the ability to sense when someone is in the front seat without a seatbelt on and that causes an annoying sound until it is resolved. It should be fairly easy to do the same with the back seats. You'd just have to add the ability for parents to set the empty car seat weight as the "no passenger" weight. Perhaps make the sound be the "panic" button sound so you hear it when you are out of the car. That or a special sound perhaps the words "child in car" could be played loudly to draw attention. There would need to be a time delay to allow for parents to walk to the back doors and get their kids out. I LOVE this idea! Even if the parents make it to the store before the time delay goes off they'd hear it then, or someone else definitely would!